The Centers for Disease Control and Prevention this week urged older Americans to get a Covid booster shot to boost their protection from the virus amid a further increase in hospitalizations, particularly among those over 70.

“In recent weeks, we have seen a precipitous and significant increase in hospital admissions for older Americans,” said CDC Director Dr. Rochelle Walensky during a public hearing before the health agency’s Committee of Independent Vaccine Experts on Thursday.

Just 43% of people aged 65 and over have had a dose of the vaccine in the past six months, and just 38% of people aged 50 to 64 have, Walensky said.

“This leaves about 60% of older Americans without the protection they may need to prevent serious illness, hospitalizations and death,” Walensky said. “We know that immunity wanes over time and we must do what we can now to protect the most vulnerable.”

Walensky said people aged 50 and over should get a fourth Covid shot and those aged 12 and over with weakened immune systems should get a fifth shot. In March, the CDC said people in these groups could receive second booster doses of Moderna and Pfizer if they so choose. The health agency on Thursday reinforced its guidance, urging people to get the shots for extra protection during the current Covid wave.

Hospital admissions have risen 25% over the past week among those aged 70 and over, with an average of more than 1,500 people in the age group being admitted each day with Covid, according to CDC data Tuesday. The US is reporting more than 100,000 new Covid infections per day on average, up 18% from the previous week, as more transmissible omicron variants sweep the US

For people age 50 and older, two doses of Pfizer and Moderna vaccines are about 50% effective six months after receiving the second shot at preventing emergency room and emergency department visits for Omicron infection, according to data published on presented at a CDC committee meeting in April. A third dose increases this protection to about 77%.

The Food and Drug Administration and CDC approved second booster shots for older Americans in March, based primarily on data from Israel. Scientists in Israel found that a fourth dose reduced the death rate from Covid by 78% in people aged 60 and over compared to those who received three shots. The study, which was not peer-reviewed, examined the health records of more than 500,000 people at Israel’s largest healthcare provider, Clalit Health Services, from January to February.

“This fourth booster dose is something that evidence we now have from Israel suggests it may reduce the risk of hospitalization and death in this elderly population,” said Dr. Peter Marks, a senior FDA official, told reporters during a call in March.

“We hope that by taking this action, we will help ensure that people can take steps to protect themselves in case we have another wave that comes through this country,” Marks said.